Fuel cells, and in particular hydrogen-oxygen fuel cells, are currently drawing increasing attention as power generating systems possessing substantially no detrimental effect against the environment. In particular, polymer fuel cells have been identified as most promising approaches for implementing fuel cell technology as they enable obtaining higher power density.
The basic element of a polymer fuel cell is the so-called “membrane electrode assembly” (MEA). The MEA comprises a polymeric membrane which consists of a proton conducting polymer and whose opposing faces are in contact with electrically conductive and catalytically active layers (also called electrode layers). Said electrode layers catalyze the oxidation of the fuel and the reduction of the oxidizing agent, and contribute to assure the necessary electrical conductivity within the fuel cell. Such layers are generally composed of the same proton conducting polymer as the membrane containing dispersed therein an active catalyst, generally a noble metal (e.g. Pt).
Perfluorinated polymers having sulfonic groups have been widely used as materials both for the polymeric membrane and the electrode layers. Nevertheless, such polymers generally comprise unstable end-groups, e.g. of —COF type and/or other irregularities and/or defects in the chain, which, during fuel cells operation, behave as decomposition-sensible moieties. Thus, during long-term fuel cells operation, the polymer of the membrane and/or of the electrode layers gradually decomposes starting from above-mentioned weakness points; these decomposition phenomena lead to the decease of the mechanical strength of the membrane and/or of the MEA, might generate pinholes, breaking, abrasion and the like, so that the power generation voltage is progressively reduced and fuel cell life-time is limited.
It is generally understood that the degradation of the membranes and/or MEAs is induced by peroxide species derived from decomposition of hydrogen peroxide formed due to the inevitable gas crossover through the membrane. Perfluorinated polymers having sulfonic groups thus are known to decompose according to the so-called “unzipping reaction”, wherein starting from —COF-type defects, generally hydrolized as —COON groups, the main chain of the polymer progressively disaggregates following the reactions scheme sketched here below:Rf—CF2COOH+.OH→Rf—CF2.+CO2+H2ORf—CF2.+.OH→Rf—CF2OH→Rf—COF+HFRf—COF+H2O→Rf—COO+HFwherein Rf represents the fluoropolymer chain.
In view of the above, stability and durability of fluoropolymers for fuel cells membranes and/or MEAs have been generally assessed with reference to Fenton tests, wherein the amount of fluoride ions released as a consequence of fluoropolymer treatment with hydrogen peroxide in the presence of iron (II) ions (catalyzing H2O2 decomposition in .OH radicals) is determined.
Several methods have been proposed in the past aiming at improving stability of fluoropolymers by reduction of defects and/or unstable end-group.
GB 1210794 (E.I. DUPONT DE NEMOURS AND COMPANY) 28 Dec. 1970 discloses a process for the stabilization of high molecular weight fluorocarbon polymers in the solid state (as particulate or pre-molding form or as molded articles) by contacting said polymers with a fluorine radical generating compound (e.g. gaseous fluorine) in the absence of oxygen. Among fluorocarbon polymers which can be stabilized according to the above-mentioned process, mention is notably made of fluorocarbon polymers having pendant —SO3H groups or precursors thereof.
US 2004242793 (DAIKIN) 2 Dec. 2004 discloses a process wherein olefinic (—CF═CF2) and/or acyl fluoride (—COF) end groups of a fluorine-containing polymer (e.g. a tetrafluoroethylene/hexafluoropropylene copolymer) are converted in stable —CF2H moieties by heat treatment in the presence of moisture at temperatures exceeding 200° C. of mixtures of said copolymers with a basic compound chosen among alkali metal or alkaline earth metal base or ammonia.
U.S. Pat. No. 4,743,658 (E.I. DUPONT DE NEMOURS AND COMPANY) 10 May 1988 discloses a process for the stabilization of tetrafluoroethylene/perfluoroalkylvinylether copolymers by fluorination of the same under the form of pellets by solid/gas reaction with fluorine gas.
Thus US 2006063903 (ASAHI GLASS COMPANY, LTD) 23 Mar. 2006 discloses a process for providing a perfluorinated polymer having sulfonic acid groups, wherein said polymer having precursors for said sulfonic acid groups (e.g. —SO2F moieties) is first submitted to a heat treatment, and then contacted with fluorine gas. The heat treatment is considered as a key step, as it enables conversion of unstable end groups in —COF moieties which can be easily converted in stable —CF3 groups by contacting with fluorine gas, so that the polymer obtained therefrom, when contacted with hydrogen peroxide in well-defined conditions exhibits a fluorine ions release of less than 0.002% ('til down to 0.001%) of the total amount of fluorine in the polymer.
Nevertheless, these processes do not provide for a suitable method for substantially reducing the number of unstable groups/defects in fluoropolymers having ion exchange groups so that the stability and durability of membranes and/or MEAs thereof is acceptable for being used in fuel cells stacks. It is important to outline that even limited traces of unstable groups, e.g. detected by non-zero fluoride emission in Fenton tests, might substantially impair durability and power generation voltage of the cell. There is thus still a need in the art for a process for stabilization of fluoropolymers having ion-exchange groups which advantageously enables substantial elimination of unstable end-group, so that virtually no F− emission in Fenton test is detected.
Also it is known in the art a process for the stabilization of amorphous polymers. Thus, EP 1256591A (AUSIMONT S.P.A.) 13 Nov. 2002 and EP 1256592 A (AUSIMONT S.P.A.) 13 Nov. 2002 disclose a process for the stabilization of amorphous perfluorinated polymers, wherein said polymer is first dissolved in a suitable solvent so as to obtain a solution having a concentration of from 0.5 to 15% by weight, and then said solution is submitted to fluorination with elemental fluorine in the presence of UV radiation. Fluoropolymers stabilized according to such method are endowed with a substantial absence of unstable polar end groups, i.e. undetectable by FT-IR spectroscopy. Among amorphous stabilized fluoropolymers obtainable by means of this process, mention is notably made of copolymers comprising recurring units derived from fluorosulfonic monomers, e.g. CF2═CF—OCF2—CF2—SO2F, CF2═CF—O—[CF2—CXF—O]n—CF2CF2—SO2F, wherein X═Cl, F or CF3 and n=1−10 and CF2═CF—OCF2—CF2—CF2—SO2F.
Nevertheless this process has limited application because it requires the fluoropolymer to be solubilized in above mentioned solvents: while amorphous materials can be solubilized with success, semi-crystalline fluoropolymers having ion-exchange groups, which possess the required mechanical properties for being used in membranes and/or MEAs for fuel cells, do not solubilize in comparable conditions. Moreover, the process according to EP 1256591 A and EP 1256592 A suffers of the disadvantages related to the burden of polymer recovery: in fact, recovery of stabilized fluoropolymer from the solution as above detailed requires onerous time- and energy-consuming procedures, such as, for instance, evaporation of the solvent and it is thus difficult to implement at the industrial level.
There is thus still a need in the art for a process for efficiently stabilizing fluoropolymers comprising ion exchange groups, which can efficiently works with materials which have no appreciable solubility and which is time- and cost-effective.